The invention relates to screw extruders and more particularly to end thrust bearing arrangements for the screws of screw extruders driven through reduction gearing.
For the mounting of the screw shaft, it has been previously proposed to provide a self-centering, self-aligning roller bearing which accommodates the axial loading of the screw. Self-aligning roller bearings have proved successful because they are evenly loaded even where there are production-occasioned deviations in alignment between screw and cylinder and also between screw shaft and drive gearing output shaft.
With increasing screw diameters, the costs of the self-aligning roller bearings, which are no longer available as a standard item for maximum screw diameters, increase. Furthermore, in the case of maximum screw diameters, the self-aligning roller bearing required to guarantee the carrying capacity has to be of considerable overall size. The large outside diameter of such a self-aligning roller bearing also results in considerable structural dimensions of the extruder housing, which ought not to exceed certain limits on grounds of space and cost.
Therefore, for large screw diameters and high operating pressures, screw extruder manufacturers have again resorted to the use of tapered roller bearings which, for an equivalent carrying capacity, have a considerably smaller outside diameter than self-aligning roller bearings. However, for them to be evenly loaded, tapered roller bearings impose exacting demands in terms of manufacturing tolerances. For large screw diameters and high working pressures, the necessary close manufacturing tolerances for the installation of tapered roller bearings onto current machine tools can no longer be achieved. Inaccuracy of alignment between the drive gearing output shaft and the screw shaft can however not be compensated by tapered roller bearings and the result is edge pressure on the rolling members of the tapered roller bearings and a consequent reduction in working life.
The invention has among its objects to provide an end thrust bearing arrangement for a screw extruder of the kind described at the outset in which damage to the end thrust bearing due to the inevitable inaccuracies in alignment of the drive gearing output shaft and the shaft of the screw extruder is prevented.